1. Area of the Art
The present invention concerns the art of microfluidics—namely the transport and processing of fluid samples on a micro-scale.
2. Description of the Background Art
This invention addresses the problem of extracting sample droplets from liquid columns by means of surface phenomena, particularly by means of electrowetting, to enable multi-staged separation and analysis functions external to the original microfluidic circuit. Digital Microfluidics (see, “Creating, Transporting, Cutting, and Merging Liquid Droplets by Electrowetting-Based Actuation for Digital Microfluidic Circuits” Sung Kwon Cho, Hyejin Moon, and Chang-Jin Kim, Journal of Microelectromechanical Systems, V. 12, NO. 1, February 2003, pp. 70-80; and M. G. Pollack, R. B. Fair, and A. D. Shenderov, “Electrowetting-based actuation of liquid droplets for microfluidic applications,” Appl. Phys. Lett., vol. 77, no. 11, pp. 1725-1726, 2000) is used as an example technology for demonstrating on chip droplet extraction. One of ordinary skill in the art will recognize that the present invention is applicable to a number of fluidic technologies where a fluid is transported in a channel, or column, and it is desired to remove a sample of the fluid without disturbing the overall transport. “Column” includes the usual devices of fluidic conveyance such as a channel, pipe, or capillary (e.g., a tube where physical walls constrain the fluid) as well as any configuration whereby a fluid is made to flow in a directed stream (e.g., a surface where surface properties constrain a flowing film of fluid to a portion of that surface). The liquid column can be formed by any arrangement that bounds or constrains the liquid flow in a particular direction or pathway, such as filling a liquid in a channel (e.g., physical walls constraining the fluid) or by applying non-uniform surface properties and effects to create a directional affinity for the liquid column (e.g. no physical walls)
In many cases the fluid column is designed not just to transport fluid but to separate and concentrate solute molecules or particles within the fluid. Many fluidic analysis systems and sensor chips (e.g., those useful in chem-bio, that is, chemistry and biology applications) utilize continuous liquid columns to transport as well as to separate and analyze fluidic samples. For example, microfluidic sensor chips often rely on walled liquid columns (microchannels) to transport and facilitate separation of a sample fluid into concentrated bands or zones of solute molecules or suspended particles by means of capillary electrophoresis (CE), Dielectrophoretic separation (DEP) and other separation techniques. Evaluation of the concentrated bands resulting is typically conducted within (or at the end of) the liquid column by using a variety of optical, electrical or chemical analytical systems. To reduce the probability of error, it is desirable to evaluate the separated band with secondary separation and analytic devices. However, these secondary instruments are often not co-located with or in close proximity to the primary analytical device; rather they may be located downstream or external to the channel-structure and thus require the transport or transfer of the concentrated band from the primary analytical device to such secondary locations. The subsequent transport of the band to these secondary evaluation stations is difficult to achieve without disturbing the column of fluid in the primary channel thereby incurring unwanted diffusion or pressure-driven dispersion of the concentrated molecules or particles. Thus, in order to integrate or operate with multi-staged separation and analysis stations, it is essential to precisely excise discrete portions of the primary fluid column and preserve the concentrated bands during transport between analysis stations.